What's the protocol when a fast is broken early? For example, fasting from 6 pm to noon the nest day normally, but one day you eat breakfast - would it be better once the fast is broken to continue eating normally for the remainder of the day, or to fast again until later in the day?

Mix it up! If you need to lean out perhaps continue the fast. If you are really pushing performance or trying to gain muscle I would EAT! I suspect that even a 15-18hr fast a few times per week may dramatically enhance insulin sensitivity which could help with gaining muscle or just staying lean. It is important to remember with the intermittent fasting you MUST on average eat as much as would have otherwise eaten. If you turn the intermittent fasting into chronic caloric restriction I think performance will suffer.